Differential pressure holding valve

ABSTRACT

A holding valve for selectively preventing discharge flow from a hydraulic motor, the valve being biased to a closed position against the force of fluid tending to discharge from one side of the motor by discharge fluid acting on a piston formed integrally with the holding valve in addition to a spring also tending to close the valve, there also being provided an opposing pilot piston area on the holding valve which when pressurized opens the valve permitting discharge flow from the associated hydraulic motor.

United States Patent Parrett 1 May 30, 1972 s41 DIFFERENTIAL PRESSURE HOLDING 3,272,085 9/1966 Hajma ..91 420 VALVE 3,359,868 12/1967 Hoffman et al.. ..9l/447 3,411,521 11/1968 Johnson ..9l/420X [721 Benm" Harm" 3,472,261 10/1969 Brannon ..91/420 x [73] Assignee: Koehring Company D Primary Examiner-Martin P. Schwadron [22] filed 1970 Assistant Examiner-Irwin C. Cohen [21] Appl. No.: 2,826 Attorney-Hofgren, Wegner, Allen, Stellman & McCord 52 us. c1 ..91/420, 91/447, 137/102 [57] ABSTRACT [51 Int. Cl ..Fl5b 13/042 A h ldi valve for selectively preventing discharge flow from [58] Field Of Search ..9l/420, 426, 447, 461, 468; a hydraulic motor. the valve being biased to a clbsed position 137/ 102 against the force of fluid tending to discharge from one side of the motor by discharge fluid acting on a piston formed in- [56] Rde'ences Cited tegrally with the holding valve in addition to a spring also UNITED STATES PATENTS tending to close the valve, there also being prm ided an opposmg pilot piston area on the holding valve wh1ch when pres- Deardorff et al ..9 X surized opens the valve permitting discharge flow from the as- 3,198,088 8/1965 Johnson et al. ..91/420 sedated hydraulic moton 3,250,185 5/1966 Tennis et a1. ..91/420 3,267,961 8/1966 Rice 137/102 12 Claims, 6 Drawing Figures g!- a2" 1, 1 a; a; 93

i "1 35 I y Patented ay 20,. 1972 3,665,810

3 Sheets-Sheet 5 DIFFERENTIAL PRESSURE HOLDING VALVE BACKGROUND OF THE PRESENT INVENTION The present invention relates generally to the control of reciprocating hydraulic motors and more particularly to a counterbalance holding valve for controlling the flow offluid relative to a hydraulic cylinder-and piston device.

In hydraulic systems where a reciprocating hydraulic motor is .employed to raise or lower a movable load, a directional control valve is conventionally employed to selectively port fluid to and from the hydraulic motor. In such devices the directional control valve is movable to a block position to prevent discharge flow from either end of the hydraulic motor to thereby attempt to hold the load stationary. However, the force of gravity acting on the load produces an increased pressure in one end of the cylinder which causes fluid to leak around the directional control valve resulting in load creepage. counterbalance holding valves have been provided in the past in an attempt to obviate this creepage problem. These prior valves have been interposed in the hydraulic circuit between the hydraulic cylinder and the directional control valve to selectively prevent flow from the cylinder when the directional control valve is in the block position. These holding valves are also capable of permitting fluid flow from one side of the cylinder when the directional control valve ports driving fluid to the other side thereof. For this purpose the holding valve is pilot operated by a pressure responsive pilot device to an open position permitting such forward flow to the hydraulic cylinder. Counterbalance holding valves have been provided also to modulate or throttle the flow from the cylinder in such a manner as to prevent cavitation in the cylinder when an external load tends to move the piston in the same direction as the incoming flow.

A disadvantage in these prior valve constructions is that since the valves are arranged to close in opposition to the force of fluid pressure in the hydraulic actuator, very strong counterbalance springs have been required to maintain the valves closed in opposition to fluid tending to discharge from the associated hydraulic actuator.

It is a primary object of .the present invention to provide a new and improved counterbalance holding valve which closes in opposition to fluid pressure within the associated hydraulic cylinder preventing discharge flow withoutrequiring the use of a high force spring to maintain the valve closed. I

SUMMARY OF THE PRESENT INVENTION In accordance with the present invention, a holding valve of the counterbalance type is provided for controlling or preventing discharge from an associated hydraulic actuator without requiring a large biasing spring by employing discharge fluid acting on a difierential piston area on the counterbalance valveto bias the valve toward its closed position. Fluid tending to discharge from the actuator thus acts not only on the end of the counterbalance valve tending to open the valve, but also on the piston area tending to close the valve. By the appropriate selection of the piston area a relatively small compression spring may be employed for biasing the counterbalance valve to its closed position.

One advantage in this differential piston arrangement, is that since the closing force on the counterbalance valve is proportional to the fluid pressure tending to discharge from the actuator the closing force increases as the fluid pressure tending to discharge from the actuator increases. This is of advantage because the force tending to open the counterbalance valve increases as the force of fluid pressure tending to discharge from the actuator increases, and therefore in prior constructions it was necessary to select a closing spring for the counterbalance valve strong enough to withstand even the higher pressures of fluid in the actuator and still hold the valve in its closed position.

In one embodiment of the present invention the differential areas on the counterbalance valves are chosen so that the hydraulic pressures acting on both ends of the valve tend to move the valve to an open position, thus reducing the required strength of the closing spring biasing the valve, similar advantages obtain, to a greater degree, if the fluid pressures acting on both ends of the valve just balance the valve, or even provide a small closing force on the valve. In the latter case, a somewhat weaker spring would be required, and such would also be desirable in the former case, but nevertheless a much smaller spring would be necessary than with conventional counterbalance holding valves.

Other objects of the present invention will be apparent from the several embodiments disclosed in more detail below.

BRIEF DESCRIPTION OF TI-IE DRAWINGS FIG. 1 is a cross section of a valve block illustrating two holding valve assemblies operable to control flow relative to both sides of a hydraulic actuator;

FIG. 2 is a fragmentary section of another holding valve according to the present invention adapted to control flow relative to one side of a hydraulic actuator;

FIG. 3 is a longitudinal section of a hydraulic actuator with still another holding valve assembly in one end plate of the actuator;

FIG. 4 is a cross section of the holding valve shown in FIG.

FIG. 5 is a cross section of another counterbalance holding valve according to the present invention; and

FIG. 6 is a fragmentary section of a still further counterbalance holding valve according to the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to the drawings and particularly FIG. 1, a counterbalance holding valve assembly 10 is illustrated, and it is generally known as a block valve assembly. The valve assembly 10 is adapted to be located in the hydraulic circuit between directional control valve and a hydraulic actuator such as a reciprocating piston and cylinder device, (not shown in FIG. 1). In some cases, as will appear in other embodiments, such as the FIGS. 3 and 4 embodiment, the holding valve assembly may be carried directly by the hydraulic actuator rather than in a separate valve block as in the FIG. 1 embodiment.

With the counterbalance valve assembly as shown in FIG. 1, with two counterbalance valves, it is possible to control and block discharge flow from both sides of the hydraulic actuator, but it should be understood that depending upon the application it may be necessary only to have one counterbalance holding valve, and thus controlling or blocking flow relative to only one side of the hydraulic actuator. One exemplary application for holding valves according to the present invention is with mobile derricks and cranes in which hydraulic actuators are employed for raising and lowering the load. In such cases the holding valves serve to control the lowering of the load under the influence of gravity and hold the load in any desired position when the control valve is placed in its neutral position.

The counterbalance valve assembly shown in FIG. 1 is seen to include a valve block housing 12 of generally rectangular construction having valve inlet-discharge passages 16 and 17 formed in side faces 19 and 20 of the valve block 12. Passages l6 and 17 are adapted to be connected to the main ports of a four-way directional control valve so that either may be pres sure or drain passages depending upon the position of the control valve (not shown).

Cylinder ports 22 and 23 are formed in the valve block faces 26 and 27, respectively, and are adapted to be connected to the opposite sides of a reciprocating piston hydraulic actuator. As the actuator may move in either direction the ports 22 and 23 may either deliver fluid to the actuator or return fluid therefrom.

Defined within the valve block 12 is a first counterbalance valve assembly 30 for controlling communication between passage 16 and cylinder port 22. Also defined within the valve block 12 is a second counterbalance valve assembly 32 for controlling communication between port 17 and cylinder port 23. The valves 30 and 32 are identical in construction and it should be understood that while a general description of both valves will be included herein, that the details of one valve apply equally with respect to the other. As may be seen from the drawings each of the counterbalance valves 30 and 32 close in opposition to fluid pressure in the cylinder ports 22 and 23 respectively. That is, a pressure increase in the cylinder ports 22 and 23 will tend to open the associated counterbalance valves 30, 32.

Referring in more detail to the construction of counterbalance valve 30 with the understanding that valve 32 is identical, a transverse stepped valve bore 36 is provided in valve block 12 and has a large cylindrical portion 37, an intermediate cylindrical portion 38 and a small cylindrical portion 39. A shoulder between cylindrical portions 38 and 39 forms a valve seat 41 for the counterbalance valve which hydraulically isolates, when the valve is closed, the cylinder port 22 from the inlet port 16.

Slidable in the stepped bore 36 is a counterbalance valve member 46 having a conical end portion 47 engageable by metal to metal contact with valve seat 41. The valve member 46 has an annular land 50 slidable in the intermediate bore portion 38 which defines, with a second annular land 52 slidable in enlarged bore portion 37, a pilot piston chamber 54 which when pressurized opens the counterbalance valve 30. Fluid is supplied to pilot piston chamber 54 through pilot passage 56 which communicates with intermediate bore 38 associated with valve 32 and communicating with inletdischarge port 17.

Extending rearwardly from the valve member 46 is a reduced piston portion 58 slidable in a stationary bushing 59 and defining a biasing piston chamber 60 which when pressurized tends to close the valve member 46 against seat 41. The effective area of the piston defined by the chamber 60 is the area between seal 62 on valve land 52 and seal 64 on valve land 58. The fluid pressure chamber 60 is pressurized by fluid in cylinder port 22 through an axially extending passage 65 in the valve member 46 which communicates at its right end with chamber 60 and at its left end opens to the small bore 39 which continuously communicates with transverse passage 69 and port 22. in this manner chamber 60 is continuously pressurized with fluid pressure in the chamber of the hydraulic motor connected to cylinder port 22 so that the closing force on the valve member 46 varies in proportion to fluid pressure in the associated actuator, increasing as fluid pressure in the actuator increases, and decreasing as fluid pressure in the fluid actuator decreases.

it should be understood that cylinder pressure also acts on left end 70 of the valve member 46 tending to open the valve 30 in opposition to fluid in chamber 60. In the embodiment shown in HO. 1 the piston area defined by chamber 60 is less than the effective piston area defined by valve end 70 so that since the pressures acting on both effective piston areas are equal there is a net hydraulic opening force on valve member 46. In opposition to the net hydraulic opening force, a coil compression spring 72 is provided in the enlarged bore portion 37 which is received within a hollow bore valve seat 73 in the right end of valve member 46. A fitting 74 is threaded into the large bore 37 and has a central bore which receives a slidable annular spring seat 76 against which spring 72 bears. Spring seat member 76 may be axially adjusted by threaded member 77 to adjust the compression of spring 72 as desired.

The counterbalance valve as thus far described serves generally to control and prevent fluid flow from the cylinder passage 22 and passage 69 across valve seat 41 to the inletdischarge passage 16. A bypass valve 80 is provided for permitting the free flow of hydraulic fluid from the port 16 to the cylinder port 22. That is, when the control valve ports fluid to port 16 it is desirable that the counterbalance valve 30 not block such flow and that it pass freely to the actuator through cylinder port 22. Toward this end the left end of the valve member 46 has a bore 82 therein which communicates with inlet-discharge port 16 through radial passages 83.'Bore 82 forms a valve seat 85 at the end thereof for a bypass valve member 87 that has a conical valve portion 89 on the right end thereof. Valve member 87 is slidable in a bore 91 in a spring seat fitting 92 threaded in main bore portion 39. Spring 93 seated within fitting 92 engages valve member 87 and biases it continuously toward valve seat 85. Upon forward flow from the main control valve, through port 16 toward port 22, valve member 87 will open permitting fluid flow from port 16 through passages 83, across valve seat 85, through port 22 to one side of the hydraulic actuator.

While the operation of the counterbalance valve 30 is believed apparent from the above description, the coordination between the counterbalance valves 30, 32 and the operation thereof will be clearer from the following description.

Assuming initially that the operator desires the actuator to move in a first direction, eg to the right, and that cylinder port 23 must be pressurized to achieve such movement of the actuator, the main directional control valve, (not shown) is shifted to connect the source of hydraulic fluid under pressure to port 17 and connect port 16 to drain. In response to this check valve associated with counterbalance valve 32 opens permitting fluid to flow out port 23 pressurizing one side of the actuator. Fluid in the other side of the actuator then tends to discharge pressurizing cylinder port 16. At substantially the same time pilot pressure from line 56 pressurizes pilot chamber 54 opening the valve member 46 and permitting discharge flow from port 22 across valve seat 41 and out port 16 to drain. The hydraulic actuator thus begins its rightward movement.

It should be understood that the efiective pilot piston area defined by pilot piston chamber 54 must be great enough to overcome the biasing force of spring 72 in addition to the hydraulic closing force produced by fluid in chamber 60 (reduced by the opening force on end 70). If at any time the main control valve is placed in its neutral position, depressurizing ports 16 and 17, the holding valve 46 will move to its closed position under the influence of spring 72 blocking flow from port 22 to port 16. Under these conditions if the pressure in either of the ports 22 or 23 increases, indicating the application of an external load to the actuator, the holding valve 30, 32 will remain closed preventing load creepage. In this regard, it will be recalled that as pressure in port 22 increases the net opening force on the valve member 46 increases since pressure in chamber 60 rises with the pressure port 22, but spring 72 is normally adjusted to provide a force slightly higher than that required to keep valve member 46 closed against the pressure induced by anticipated external loads.

For leftward movement of the actuator, the main control valve is shifted to its opposite position pressurizing port 16 and connecting port 17 to drain. Under these conditions check valve 80 will open permitting flow thereacross to port 22 pressurizing the side of the actuator which tends to produce leftward movement of the actuator resulting in a tendancy to discharge fluid from the other side of the actuator increasing pressure in port 23. Pilot fluid at the same time flows through pilot passage 56 pressurizing pilot chamber 54' and opening counterbalancing valve member 46' and permitting discharge flow from the port 23 to the inlet-discharge port 17 which is then connected across the main control valve to drain. The spring bore 73 continuously communicates with the inletdischarge port 16 through central bore 94 in valve member 46.

It may thus be seen that the counterbalance valve assembly 10 controls rapid movement of the load or actuator in either direction by controlling discharge flow from the actuator and locks the actuator in any desired position without load creepage whenever the main hydraulic control valve is placed in its neutral position.

A similar counterbalancing holding valve assembly is shown in FIG. 2 except that the bypass check valve is piloted within the main counterbalance valve member and the biasing piston area on the valve member is defined by the main spring seat portion thereof rather than by an annular skirt portion of the valve member. Toward this end and with reference to FIG. 2 a valve housing 112 is provided having an inlet passage 116 adapted to be selectively connected to a pressure or return line across the main control valve, and a cylinder port 122 connected to one side of a hydraulic actuator. Defined within housing 112 is a stepped valve bore 136 having an enlarged bore portion 137, an intermediate bore portion 138 and a narrow bore portion 139. The shoulder between bore portions 138 and 139 defines a valve seat 141. Slidable in bore 136 is a valve member 146 having a conical surface 147 at the left end thereof engageable by metal to metal contact with valve seat 141, to block flow from cylinder port 122 to inlet-discharge port 1 16 in a manner similar to the FIG. 1 embodiment.

The valve member 146 has an annular valve land 150 slidable in bore 138 and an annular valve land 152 slidable in enlarged bore portion 137 which together define therebetween a pilot chamber 154 which when pressurized exerts a force on valve member 146 sufficient to open the valve.

For the purpose of urging the valve member 146 toward its closed position against seat 141 a coil compression spring 172 is provided in an enlarged bore 173 in the right end of valve member 146. Spring 172 is seated within a cup shaped spring seat 176 which may be axially adjusted by a threaded abutment 177 in fitting 174 threaded in housing 112. In distinction to the FIG. 1 embodiment, the biasing pressure chamber is defined by bore 173 as indicated at 160. Chamber 160 continuously communicates with fluid in the cylinder port 122 through axial passage 165 in valve member 146 opening to the bore 139. Chamber 196 formed by the annular right end of valve member 146 within enlarged bore 137 continuously communicates with inlet-discharge 116 through axial passage 194.

For the purpose of permitting free flow from the inletdischarge port 116 to cylinder port 122, check valve assembly I80 is provided including a valve member 187 having a conical surface 189 engageable with a valve seat defined by bore 182 in the main valve member which continuously communicated with valve inlet 116 through radial passages 183. Check valve 180 has a cylindrical pilot member 161 slidable in bore 182 so that the entire check valve assembly 180 is carried by the counterbalance valve member 146. A spring 193 in the left end of bore 136 biases the check valve member 180 to its closed position shown in FIG. 2.

Since the operation of the counterbalance valve 100 shown in FIG. 2 is identical to the operation of the counterbalance valves 30 and 32 shown in FIG. 1 a repetition of the operation thereof is not believed necessary.

One difference not noted above with respect to the FIG. 1 and 2 embodiments is that in the FIG. 2 embodiment the effective piston area defined by chamber 160 tending to close the valve is approximately equal to the effective piston area defined by the left end of the valve tending to open the valve. Thus, the hydraulic forces acting on the valve are approximately equal (in the absence of pilot pressure) regardless of the pressure in cylinder port 122, and the spring 172 provides the necessary force to maintain the valve closed. Further, the efiective pilot piston area defined by chamber 154 is sized so that sufficient force is exerted on the valve, when desired, to overcome the force of spring 172 which will be sufficient to permit opening of valve member 146.

Referring to FIGS. 3 and 4 a further modification of the present holding valve is shown, wherein a holding valve assembly 200 is shown mounted directly in end plate 212 of a reciprocating hydraulic actuator 211. The counterbalance valve assembly 200 is generally similar to that shown in FIG. 2, except that check valve assembly 280 seats within housing 213 rather than in counterbalance valve member 246. Toward this end the housing member 213 is provided with a valve seat 285 against which the valve member 280 seats. A central opening 282 in the valve member 246 receives a cylindrical pilot portion 261 of the check valve member 280 for the purpose of piloting the check valve 280. Spring 193 is received within valve chamber 160 and reacts against the upper end of pilot 261 to urge the check valve 280 to its seated position shown in the drawing.

The operation of the valve 200 is the same as the valve shown in FIG. 2 but a brief. description is believed helpful in view of the illustration of the actuator 211 in FIGS. 3 and 4. The actuator 211 has a first port 218 and a second port 225 which when pressurized causes leftward movement of the piston 240. Assuming port 216 to be pressurized and port 218 connected to drain, check valve 280 will open permitting flow through cylinder ports 222 and 225 to the right side of piston 240. Assuming only a single counterbalance valve 200 is provided, the resulting flow from the port 218 will pass through a conduit 270 directly to a directional control valve rather than across another counterbalance valve. In these conditions the counterbalance valve member 246 will remain in its closed position. When it is desired to move the piston 240 in its rightward direction of movement, the main control valve is shifted to its opposite position pressurizing port 218 and connecting port 216 to drain. Underthese conditions fluid will flow into port 218 pressurizing the left side of the piston, tending to urge piston 240 to the right increasing the pressure in port 222 and under the counterbalance valve member 246. At substantially the same time pilot fluid will flow through passage 254 into pilot chamber 256 opening the valve member 246 against the opposing forces as noted with respect to the FIG. 2 construction. Fluid may then discharge from the right side of the actuator 211 across valve seat 241, through passage 242 and chamber 243 into port 216.

Counterbalance valve 300 shown in FIG. 5 is in effect a combination of the counterbalance valve assemblies shown in FIGS. 1 and 2. It is similar to the FIG. 1 construction in that an annular biasing chamber 360 is provided for the purpose of biasing the valve member to a closed position which communicates with the cylinder port 22 through an axial passage 365 in the valve chamber. However, rather than providing a housing piloted bypass valve as in the FIG. 1 embodiment a valve piloted bypass valve 380 is provided which has a pilot portion 361 slidable in a central bore 382 in main counterbalance valve member 346. Upon pressurization of inlet port 316 the valve member 380 will move away from its seated position shown in opposition to the biasing force of spring 393 permitting forward flow from port 316 to cylinder port 322. Spring 393 is seated in the chamber associated with the main biasing spring 372 and reacts against a snap ring 368 mounted on the extension from pilot portion 361. As with the other embodiments the entire left end of the main valve member 346,

i.e. the area within the seat 341 is subjected to cylinder pressure in port 322.

A still further embodiment is illustrated in FIG. 6, and this construction is substantially the same as that described with reference to FIGS. 3 and 4 except for certain variations. As with the FIG. 3 embodiment the check valve 480 is seated within a housing member 413 although it is piloted as at 482 within the main valve member 446. A further distinction is that the main valve member 446 has three internally stepped portions 470, 471 and 472, the latter being the biasing portion. The main compression spring 473 for the valve is located within large counterbore 474 and reacts against a ring 476 in the large bore. An axially adjustable pilot member 477, shiftable through an adjustable seat 483, engages an annular spring seat 484 to adjustably bias the check valve spring 493 and main spring 473.

The fluid pressure biasing chamber in the valve assembly 400 is defined by the bore portions 471 in valve member 446 along with the spring seat 484. For the purpose of communicating this chamber with the cylinder port 422, passage 465 is provided in the check valve. Valve 400 operates in the same manner as valve 200 and a detailed description thereof is not believed necessary.

I claim:

1. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, discharge passage means in said housing means adapted to receive fluid from said hydraulic load, holding valve means in said discharge passage means for selectively preventing discharge flow from said hydraulic load, and means for biasing said holding valve means to a closed position, including means biasing said holding valve means to a closed position in opposition to fluid pres sure in the discharge passage means and the hydraulic load, fluid pressure in said discharge passage means tends to open the holding valve, said means biasing said holding valve means to a closed position including means responsive to fluid pressure in said discharge passage means on the hydraulic load side of said holding valve means, and bypass valve means for supplying fluid to the hydraulic load.

2. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, first discharge passage in said housing means for receiving fluid from the hydraulic load, second discharge passage means in said housing means for conveying fluid from the housing means, holding valve means selectively blocking flow from said first discharge passage means to said second discharge means, fluid pressure in said first discharge passage means tends to open said holding valve means, said holding valve means being constructed to close in opposition to the force of fluid in the first discharge passage means, means biasing said holding valve means to a blocking position including means responsive to fluid pressure in said first discharge passage means, and means for selectively opening said holding valve means to provide communication between said first discharge passage means and said second discharge passage means.

3. A counterbalance holding valve for controlling flow relative to a hydraulic load, comprising; housing means, first discharge passage in said housing means for recovering fluid from the hydraulic load, second discharge passage means in said housing means for conveying fluid from the housing means, counterbalance holding valve means selectively blocking flow from said first discharge passage means to said second discharge passage means, fluid pressure in said first discharge passage means tends to open said holding valve means, said holding valve being positioned to close in opposition to the force of fluid in the first discharge passage means, means biasing said counterbalance holding valve means to a blocking position including means responsive to fluid pressure in said first discharge passage means, and means for selectively opening said counterbalance holding valve means to provide communication between said first discharge passage means and said second discharge passage means.

4. A counterbalance holding valve as defined in claim 3 including spring means biasing said counterbalance holding valve means to a closed position in opposition to the force of fluid pressure in said first discharge passage means.

5. A counterbalance holding valve as defined in claim 3 including a bypass valve in said housing means coaxial with said counterbalance holding valve means for permitting flow from said second discharge passage to said first discharge passage means to pressurize the hydraulic load.

6. A holding valve as defined in claim 2 wherein said means for biasing said holding valve means to a closed position including piston means associated with said holding valve and positioned when pressurized to tend to close said holding valve means, and fluid passage means communicating said piston means with said first passage means.

7. A holding valve as defined in claim 2, including a bypass valve associated coaxially with respect to said holding valve to said first discharge passage means, said bypass valve means having seat means in said counterbalance holding valve means.

9. A counterbalance holding valve as defined in claim 3, wherein said means biasing said holding valve means to a blocking position includes an annular peripheral piston portion on said counterbalance holding valve means, and passage means communicating with said piston portion and extending axially through said counterbalance holding valve means to said first discharge passage means.

10. A counterbalance holding valve as defined in claim 3, wherein said means biasing said holding valve means to a blocking position includes a central circular piston portion on said counterbalance holding valve means, and passage means communicating with said piston portion and extending axially through said counterbalance holding valve means to said first discharge passage means.

1 1. A counterbalance holding valve assembly, comprising: a housing, first passage means in said housing adapted to be connected selectively to a source of fluid under pressure or drain, second passage means in said housing adapted to be connected to one side of a fluid actuator, a valve seat in said housing communicating said first and second passage means, a valve bore in said housing coaxial with said valve seat, a counterbalance holding valve slidable in said valve bore and having a conical end portion directly engageable with said valve seat, fluid pressure in said second passage means tends to open said holding valve, said counterbalance holding valve being movable to a closed position in opposition to fluid pressure in said second passage means, piston meanson said counterbalance holding valve when pressurized tending to urge said valve to its seated position, axially extending passage means in said counterbalance holding valve communicating said piston means with said second passage means so that fluid in said second passage means tends to close the counterbalance holding valve, spring means in said valve bore urging said counterbalance valve means toward its closed position, pilot piston means on said counterbalance holding valve when pressurized opening said counterbalance holding valve against the force of fluid acting on said piston means, and bypass valve means in said valve bore permitting fluid flow from said first passage means to. said second passage means to pressurize said one side of the hydraulic actuator.

12. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, discharge passage means in said housing means adapted to receive fluid from said hydraulic load, holding valve means in said discharge passage means for selectively preventing discharge flow from said hydraulic load, and means for biasing said holding valve means to a closed position, including means biasing said holding valve means to a closed position in opposition to fluid pressure in the discharge passage means and the hydraulic load, fluid pressure in said discharge passage means tends to open the holding valve, said means biasing said holding valve means to a closed position including means responsive to fluid pressure in said discharge passage means on the hydraulic load side of said holding valve means. 

1. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, discharge passage means in said housing means adapted to receive fluid from said hydraulic load, holding valve means in said discharge passage means for selectively preventing discharge flow from said hydraulic load, and means for biasing said holding valve means to a closed position, including means biasing said holding valve means to a closed position in opposition to fluid pressure in the discharge passage means and the hydraulic load, fluid pressure in said discharge passage means tends to open the holding valve, said means biasing said holding valve means to a closed position including means responsive to fluid pressure in said discharge passage means on the hydraulic load side of said holding valve means, and bypass valve means for supplying fluid to the hydraulic load.
 2. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, first discharge passage in said housing means for receiving fluid from the hydraulic load, second discharge passage means in said housing means for conveying fluid from the housing means, holding valve means selectively blocking flow from said first discharge passage means to said second discharge means, fluid pressure in said first discharge passage means tends to open said holding valve means, said holding valve means being constructed to close in opposition to the force of fluid in the first discharge passage means, means biasing said holding valve means to a blocking position including means responsive to fluid pressure in said first discharge passage means, and means for selectively opening said holding valve means to provide communication between said first discharge passage means and said second discharge passage means.
 3. A counterbalance holding valve for Controlling flow relative to a hydraulic load, comprising; housing means, first discharge passage in said housing means for recovering fluid from the hydraulic load, second discharge passage means in said housing means for conveying fluid from the housing means, counterbalance holding valve means selectively blocking flow from said first discharge passage means to said second discharge passage means, fluid pressure in said first discharge passage means tends to open said holding valve means, said holding valve being positioned to close in opposition to the force of fluid in the first discharge passage means, means biasing said counterbalance holding valve means to a blocking position including means responsive to fluid pressure in said first discharge passage means, and means for selectively opening said counterbalance holding valve means to provide communication between said first discharge passage means and said second discharge passage means.
 4. A counterbalance holding valve as defined in claim 3 including spring means biasing said counterbalance holding valve means to a closed position in opposition to the force of fluid pressure in said first discharge passage means.
 5. A counterbalance holding valve as defined in claim 3 including a bypass valve in said housing means coaxial with said counterbalance holding valve means for permitting flow from said second discharge passage to said first discharge passage means to pressurize the hydraulic load.
 6. A holding valve as defined in claim 2 wherein said means for biasing said holding valve means to a closed position including piston means associated with said holding valve and positioned when pressurized to tend to close said holding valve means, and fluid passage means communicating said piston means with said first passage means.
 7. A holding valve as defined in claim 2, including a bypass valve associated coaxially with respect to said holding valve means to permit flow from said second discharge passage means to said first discharge passage means, said bypass valve means having seat means in said housing means and a stem portion slidable in said counterbalance holding valve means.
 8. A holding valve as defined in claim 2, including a bypass valve associated coaxially with respect to said holding valve means to permit flow from said second discharge pump means to said first discharge passage means, said bypass valve means having seat means in said counterbalance holding valve means.
 9. A counterbalance holding valve as defined in claim 3, wherein said means biasing said holding valve means to a blocking position includes an annular peripheral piston portion on said counterbalance holding valve means, and passage means communicating with said piston portion and extending axially through said counterbalance holding valve means to said first discharge passage means.
 10. A counterbalance holding valve as defined in claim 3, wherein said means biasing said holding valve means to a blocking position includes a central circular piston portion on said counterbalance holding valve means, and passage means communicating with said piston portion and extending axially through said counterbalance holding valve means to said first discharge passage means.
 11. A counterbalance holding valve assembly, comprising: a housing, first passage means in said housing adapted to be connected selectively to a source of fluid under pressure or drain, second passage means in said housing adapted to be connected to one side of a fluid actuator, a valve seat in said housing communicating said first and second passage means, a valve bore in said housing coaxial with said valve seat, a counterbalance holding valve slidable in said valve bore and having a conical end portion directly engageable with said valve seat, fluid pressure in said second passage means tends to open said holding valve, said counterbalance holding valve being movable to a closed position in opposition to fluid pressure in said second passage means, piSton means on said counterbalance holding valve when pressurized tending to urge said valve to its seated position, axially extending passage means in said counterbalance holding valve communicating said piston means with said second passage means so that fluid in said second passage means tends to close the counterbalance holding valve, spring means in said valve bore urging said counterbalance valve means toward its closed position, pilot piston means on said counterbalance holding valve when pressurized opening said counterbalance holding valve against the force of fluid acting on said piston means, and bypass valve means in said valve bore permitting fluid flow from said first passage means to said second passage means to pressurize said one side of the hydraulic actuator.
 12. A holding valve for controlling flow relative to a hydraulic load, comprising; housing means, discharge passage means in said housing means adapted to receive fluid from said hydraulic load, holding valve means in said discharge passage means for selectively preventing discharge flow from said hydraulic load, and means for biasing said holding valve means to a closed position, including means biasing said holding valve means to a closed position in opposition to fluid pressure in the discharge passage means and the hydraulic load, fluid pressure in said discharge passage means tends to open the holding valve, said means biasing said holding valve means to a closed position including means responsive to fluid pressure in said discharge passage means on the hydraulic load side of said holding valve means. 